net/socks5/tssocks: add a SOCKS5 dialer type, method-ifying code

https://twitter.com/bradfitz/status/1409605220376580097

Prep for #1970, #2264, #2268

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/socks5/tssocks/tssocks.go

@@ -27,34 +27,53 @@ import (
 //
 // If ns is non-nil, it is used for dialing when needed.
 func NewServer(logf logger.Logf, e wgengine.Engine, ns *netstack.Impl) *socks5.Server {
-	srv := &socks5.Server{
-		Logf: logf,
+	d := &dialer{ns: ns}
+	e.AddNetworkMapCallback(d.onNewNetmap)
+	return &socks5.Server{
+		Logf:   logf,
+		Dialer: d.DialContext,
 	}
-	var (
-		mu  sync.Mutex // guards the following field
-		dns netstack.DNSMap
-	)
-	e.AddNetworkMapCallback(func(nm *netmap.NetworkMap) {
-		mu.Lock()
-		defer mu.Unlock()
-		dns = netstack.DNSMapFromNetworkMap(nm)
-	})
-	useNetstackForIP := func(ip netaddr.IP) bool {
-		// TODO(bradfitz): this isn't exactly right.
-		// We should also support subnets when the
-		// prefs are configured as such.
-		return tsaddr.IsTailscaleIP(ip)
+}
+
+// dialer is the Tailscale SOCKS5 dialer.
+type dialer struct {
+	ns *netstack.Impl
+
+	mu  sync.Mutex
+	dns netstack.DNSMap
+}
+
+func (d *dialer) onNewNetmap(nm *netmap.NetworkMap) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	d.dns = netstack.DNSMapFromNetworkMap(nm)
+}
+
+func (d *dialer) resolve(ctx context.Context, addr string) (netaddr.IPPort, error) {
+	d.mu.Lock()
+	dns := d.dns
+	d.mu.Unlock()
+	return dns.Resolve(ctx, addr)
+}
+
+func (d *dialer) DialContext(ctx context.Context, network, addr string) (net.Conn, error) {
+	ipp, err := d.resolve(ctx, addr)
+	if err != nil {
+		return nil, err
 	}
-	srv.Dialer = func(ctx context.Context, network, addr string) (net.Conn, error) {
-		ipp, err := dns.Resolve(ctx, addr)
-		if err != nil {
-			return nil, err
-		}
-		if ns != nil && useNetstackForIP(ipp.IP()) {
-			return ns.DialContextTCP(ctx, addr)
-		}
-		var d net.Dialer
-		return d.DialContext(ctx, network, ipp.String())
+	if d.ns != nil && d.useNetstackForIP(ipp.IP()) {
+		return d.ns.DialContextTCP(ctx, ipp.String())
+	}
+	var stdDialer net.Dialer
+	return stdDialer.DialContext(ctx, network, ipp.String())
+}
+
+func (d *dialer) useNetstackForIP(ip netaddr.IP) bool {
+	if d.ns == nil {
+		return false
 	}
-	return srv
+	// TODO(bradfitz): this isn't exactly right.
+	// We should also support subnets when the
+	// prefs are configured as such.
+	return tsaddr.IsTailscaleIP(ip)
 }